Authorities in northern Nigeria say 276 teenage girls abducted from a school last month are still unaccounted for.

Borno state police and intelligence officials raised their estimate of the missing late Thursday after complaints from parents who said earlier numbers were too low.

Police commissioner Tanko Lawan said it was "really difficult" to say how many girls were actually missing because students from other schools were at the site that day to take final exams.

The mass abduction took place on April 15, after suspected Islamist militants attacked the town of Chibok.

Lawan said 53 of the abducted girls have escaped.

A state security official, Ahmed Abdullahi, said the number of girls known to have been kidnapped may continue to change as parents from neighboring villages come forward and report their daughters as missing. He said "it is not the number that matters because all the girls are very important."

Relatives and supporters of the missing girls are using social media sites to raise international awareness of their plight.

On Twitter, some used the hash tag #BringBackOurGirls to voice frustration over what they view as a slow response from Nigerian security forces. Some have called for President Goodluck Jonathan's resignation.

Advocates are also using social media to publicize overseas rallies in support of the girls.

On Wednesday and Thursday, Nigerians held protests in Abuja and elsewhere to demand that the government do more to find and rescue the schoolgirls.

Initial reports said the girls had been taken into the massive Sambisa forest. Reports this week said some of the students have been forced into so-called marriages with their captors or taken to other locations.

There has been no claim of responsibility for the attack and kidnappings. However, the assault is similar to others carried out by Boko Haram, an Islamist militant group that has been active in the region.

Borno is one of three states where the government declared a state of emergency last May. Nigeria's military has launched operations meant to crush Boko Haram, but large-scale attacks have continued.